1. Field of the Invention
The present invention relates to methods and apparatus for transmitting a load to the plurality of load supporting members, so that each member is supporting its share of the total supported load. More particularly, the present invention relates to a drill string compensator with improved transloaders, and to a method for preloading the compensator to eliminate any slack in the load supporting members.
2. Description of the Background
Those skilled in the offshore oil, gas, or geothermal drilling industry appreciate that drill string compensators are commonly employed to compensate for the vertical motion imparted to a marine drilling vessel by normal wave action. During drilling or when conducting other undersea operations, this compensation is necessary and/or beneficial so that the drill string remains stationary with respect to the seabed, and is thus not responsive to the wave action. Marine vessels suitable for drilling commonly employ a derrick fixed to the deck, with a crown block at the upper end of the derrick and a traveling block suspended from the crown block by an array of cable lines. The cable lines extend between the sheaves of the blocks and then to the drawworks which is controllably rotated to raise or lower the traveling block. A traveling block has a hook to which an elevator or other suitable equipment is connected for grasping the top portion of the drill string, which extends downward through the water and into the seabed. A fluid actuated compensator system provides a reciprocal motion which allows the hook to be maintained at a fixed location relative to the seabed floor as the traveling block heaves with the wave action imparted to the vessel. Such compensators require that the weight of the compensation apparatus be supported by the same cable line which supports the traveling block from the crown block.
A pair of hydraulic or pneumatic piston and cylinder assemblies with multiple chain arrays typically provide the actual interconnection between the main frame and the movable frame of the compensator. The present invention relates to techniques for adjusting those chain arrays to ensure that each of the parallel load supporting members in the drill string compensator supports its intended share of the total supported load. Proper adjustment of the load supporting members in the compensator thus avoids an overload of one of the chains, which may otherwise result in a significantly decreased useable life for the chains, or could result in catastrophic failure of one or more of the chains.
Techniques have heretofore been employed for adjusting the load supporting members of a drill string compensator so that they each support approximately the same load. More refined techniques for accurately adjusting these load support members are disclosed in U.S. Pat. No. 4,723,805. The methods and apparatus of the '805 patent thus represent a significant improvement over techniques previously utilized for attempting to ensure that the weight of the drill string is shared equally by each of the plurality of load supporting members in the drill string compensator.
In spite of the advancements made by providing a transloader for a drill string compensator, the acceptance and use of a compensator with a transloader has heretofore been limited by a number of problems relating to the transloader. Drilling operators continue to have concerns over the additional weight added to the drill string compensator by the transloader. Also, improved techniques are preferred to facilitate the installation and maintenance of the transloader, and to improve the safety and operation of a drill string compensator with a transloader. Moreover, prior art equipment of the type described in the '805 patent does not adequately insure that the drill string compensator cylinders do not "bottom out" before the compensator is in position so that it can be locked out of service.
These disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for adjusting the tension in the plurality of chains of a drill string compensator to provide a uniform distribution of the supported load on each of the plurality of supporting members.